Fluoroelastomers and in particular perfluoroelastomers such as those described in “Modem Fluoropolymers”, edited by John Scheirs, Wiley Science 1997, offer excellent protection against high service temperatures and are resistant to a wide variety of chemical reagents. Fluoroelastomers are generally divided in two main classes, namely those that are based on vinylidene fluoride (VF2) and those that do not contain VF2 and are based on perfluoro vinyl ethers, and tetrafluoroethylene (TFE) and/or chlorotrifluoroethylene (CTFE).
Fluoropolymers of VF2 and a perfluorovinyl ether may be used for producing fluoroelastomers having a low Tg. The fluoroelastomers having a particularly low Tg are generally obtained with highly levels of VF2, e.g. of 50 to 80 mol %. Unfortunately, the use of VF2 in fluoroelastomers generally reduces the chemical and heat resistance of the fluoropolymer and in particular, such fluoroelastomers may be prone to swelling when brought in contact with organic solvents as is for example the case in fuel system application. Thus, although a very low Tg can be achieved, this advantage is accompanied with a decrease of other physical properties.
Fluoropolymers may also contain VF2 and certain perfluorinated allyl ethers. The fluoropolymers are cured using a bisphenol cure composition and low Tg elastomers are obtained. Typically, the amount of VF2 used in the copolymers is 50 mol % or more. Thus, the low Tg fluoroelastomers suffer from the same disadvantages as disclosed above.
Fluoropolymers may also comprise fluorinated allyl ether repeating units. Nitrile functionalized fluorinated allyl ethers can be used to make fluoropolymers that can be cured to fluoroelastomers through a cure reaction involving the nitrile groups. No properties of such fluoroelastomers are disclosed however.
Krytox™ perfluoroalkyl polyether oils may be added to fluoropolymers to lower Tg. However, these plasticizers can be extracted by solvents over time.
Perfluoro-terpolymers may also consist of tetrafluoroethylene, perfluoromethyl vinylether and at least 3 mol % of certain long chain vinylethers. The long-chain vinylether lowers the Tg significantly, however the incorporation is rather difficult. Therefore, one has to run the polymerization in perhalogenated solvents (e.g. R 113) or in aqueous emulsion polymerization in the presence of fluorinated alcohols. The disadvantages of these systems are: the use of perhalogenated solvents (e.g. R 113) is often critical due to environmental concerns and the removal of the fluorinated alcohols is often very difficult because they act as swelling agents.
Aqueous emulsion polymerization may be used to copolymerize tetrafluoroethylene, hexafluoropropylene and perfluorovinylethers of the formula CF2═CFO—(CF2CFXO)m—Rf wherein X is F or CF3, m is 1 to 50 and Rf is a perfluoroalkyl group. Although these fluoroelastomers have a low Tg, their method of making involves polymerization times of up to 28 hours, making their manufacturing expensive. Similarly, the aqueous emulsion polymerization of tetrafluoroethylene and certain perfluorovinylethers required in polymerization times on the order of 30 hours.
It would now be desirable to find further fluoropolymers for making fluoroelastomers. In particular, it would be desirable to find fluoropolymers that can be manufactured in a cost effective way. Desirably the fluoropolymers are easy and conveniently processable in the making of fluoroelastomer articles therefrom. It would further be desired that fluoroelastomers made from such fluoropolymers have good or excellent mechanical and physical properties, including for example excellent chemical and heat resistance and low or no swelling with organic solvents. Desirably, the fluoropolymers would allow producing fluoroelastomers having a low Tg for example as may be required for fuel system applications in for example engines of airplanes.